I have tried Al doped nickel oxide by spray pyrolysis. Al doping does not improve the value of capacitance of NiO. What is the reason behind this? Suggest me the way.
Which another elements have to try to improve the capacitance of NiO.
Understanding basics of doping strategies is important. While studying electrical properties, you dope primarily to achieve either better conductivity or better polarization or dielectric attributes. If you want to increase conductivity, you should think of adding more electrons or holes in the system. Based on the p-type or n-type nature of the mother compound, you have to look for a dopant of suitable oxidation state, which can increase the electron-density or hole-density appropriately. So, you see, if you want to increase conductivity, the oxidation state of the dopant and the mother cation should not match.
On the other hand, if you want to increase polarization, then obviously you will try to avoid hiking free carrier density. Therefore, the oxidation state of the dopant and mother cation should match, otherwise leakage conductance will grow via the extra carriers.
In NiO, Ni is in +2 oxidation state. So, if you dope Al3+ at this site, it will give you extra electrons, which might give you better conductivity, but not a good choice to enhance dielectric constant. Although this thumb rule might change in special cases, where you might achieve better conductivity and dielectric constant via a single doping, but initially it's better to not think in that line. So, here you may try Ca, Mg, Zn etc.
Another suggestion, be very cautious, if you want to dope a transition metal with more than one stable oxidation states, because in that case it might be difficult to predict, which oxidation state will be active after the substitutional doping.
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